1. Field of the Invention
This application relates to a multistage vacuum pump unit and an operation method thereof, whereby energy-consumption-saving (a power requirement reduction) is achieved while the unit is started and an obtained vacuum state is under a low vacuum condition.
2. Description of the Related Arts
In conventional vacuum pumps, each pump requires maximum power of its own while the pump is started and an obtained vacuum state is under a low vacuum condition. As the gas pressure in a to-be-vacuumized container is lowered, power demand is gradually reduced. Thus, the power demand becomes minimal when an obtained vacuum pressure reaches a reference vacuum pressure, or an expected vacuum pressure.
Some of the conventional vacuum pumps adopt a speed control for themselves when the obtained pressures become closer to ultimate (goal) pressures (high vacuum pressures), so that the above-described energy-consumption-saving (a power requirement reduction) is achieved; however, according to this manner, a little can be expected as to the power reduction during the low vacuum operation.
In other words, according to the above manner, not only the above-described maximum power but also a starting power (such as an overshooting of a power requirement) are required when the vacuum pumps are started, for instance, in the case where the gas pressure in a to-be-vacuumized container is equal to a level of an atmospheric pressure; thus, a driving motor of a large power rating becomes necessary, and the cost of facilities is increased. Accordingly, when the energy-consumption-saving (a power requirement reduction) during the low vacuum operation after starting a vacuum pump is achieved, the cost of facilities can be reduced thanks to the power reduction effect as to the vacuum pump.
Japanese Laid-open patent application No. S62-48979 discloses a scroll compressor provided with a structure so as to reduce a power requirement reduction in starting the compressor. The compressor comprises a stationary scroll and an orbiting scroll that form a first compression space and a second compression space. When the compressed gas pressure in the first space exceeds that in the second space, the compressed gas in the first space is discharged into the second space by means of at least one open-and-close valve. Further, the gas discharged into the second space is discharged outside when the second space is in communication with a discharging port that connects the second space toward outside of the scroll compressor.
If the structure disclosed in Japanese Laid-open patent application No. S62-48979 is straightly applied to the multistage vacuum pump as a structure thereof, and a highly compressed gas generated in a preceding stage vacuum pump is discharged into a subsequent stage vacuum pump, then the gas pressure in a gas flow passage of the succeeding stage vacuum pump becomes excessively high and a power requirement reduction can not be achieved. On the contrary, high heat may be emitted in the subsequent stage vacuum pump.
Japanese Laid-open patent application No. H8-270582 discloses a scroll type two-stage vacuum pump that overcomes the difficulties in implementing the improvements of the patent reference 1 as well as in reducing the emitted heat in a viscous gas flow region of a low vacuum; the structure of the reference 2 is provided with a by-pass passage that connects a discharge outlet of a preceding stage scroll pump, to an intermediate passage between a discharge outlet of the preceding stage scroll pump and a suction inlet of a succeeding stage scroll pump, while the intermediate passage is provided with a pressure control valve that closes the passage when the pressure in the intermediate passage becomes lower than a predetermined pressure.
In the above-described structure, when the pressure in a to-be-vacuumized container is closer to an atmospheric pressure at an early stage in starting the vacuum pump, and the discharge pressure of the preceding stage scroll pump is higher than a predetermined pressure, for instance, an ambient pressure, then the above described pressure control valve is opened, and the high pressure gas at the preceding stage scroll pump outlet is discharged outside through the by-pass passage without being delivered to the succeeding stage scroll pump. In this manner, an excessive pressure accompanied by high-heat in the succeeding stage scroll pump can be prevented; in addition, a durability-deterioration as well as a seizure-problem due to the high heat in the vacuum pump can be prevented.
According to Japanese Laid-open patent application No. H8-270582, the disclosed structure comprises:
an intermediate passage between a discharge outlet of a preceding stage scroll pump and a suction inlet of the succeeding stage scroll pump;
a by-pass passage that connects a discharge outlet of a succeeding stage scroll pump, to an intermediate passage. Hereby, it is noted that an outlet pressure of the succeeding stage scroll pump is closer to an atmospheric pressure, since the compressed gas is released toward outside at the outlet.
Therefore, a pressure difference is developed between the outlet of the succeeding stage scroll pump and a place along the gas passage inside the vacuum pump in response to a produced high vacuum in a to-be-vacuumized container (a closed container as a load absorbing element); thus, there arises a risk that outside gas or air flows back inside of the vacuum pump, through the by-pass passage and the pressure control valve. The potential back-flow causes a difficulty that an operational efficiency of the vacuum pump is lowered.